In one sense Chuck Westfall is correct in stating that the FS4000's Dmax should be advertised as 4.2 not 3.4. To the best of my knowledge there is no standard independently-verifiable Dmax test. In the absence of such a test, other manufacturers claim they have the maximum theoretical Dmax for a given bit-depth, so Canon should too.

When you put a 12-bit analogue-to-digital converter in your scanner, as I understand it, the best Dmax it could possibly produce would be 3.4. Similarly, the best Dmax a 14-bit ADC could produce would be 4.2. In fact, the FS-4000 probably does achieve a 4.2 Dmax ... IF you ignore noise. By this I mean that you could probably distinguish between two adjacent dark regions in a slide in which the difference in colour/tone is only one bit in 14. The problem would be that the number of noisy pixels would mar the useability of your results.